Fluorescent ink compositions comprising functionalized fluorescent nanocrystals

ABSTRACT

A fluorescent ink composition comprising functionalized fluorescent nanocrystals, an aqueous-based ink carrier comprising water or a water-based solution, and a binder. Also provided are methods of providing an image or a security mark on a substrate for subsequent identification by applying a fluorescent ink composition onto the substrate

The present application is a continuation of U.S. application Ser. No.09/755,407 filed Jan. 4, 2001, now U.S. Pat. No. 6,576,155, which is acontinuation-in-part of U.S. patent application Ser. No 09/436,145,filed Nov. 9, 1999, which is a continuation in part U.S. patentapplication Ser. No. 09/372,729, filed Aug. 11, 1999, now U.S. Pat. No.6,114,038, which claims priority to and the benefit of the provisionalU.S. patent application Ser. No. 60/107,829 filed Nov. 10, 1998, thedisclosures of which are herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to water-based printing inks; and moreparticularly to fluorescent ink compositions useful in variousprinting~methods such as ink jet printing.

BACKGROUND OF THE INVENTION

Ink jet printing is a standard and preferred method for printing onto asubstrate, wherein a stream of ink droplets are directed from a printingdevice to a surface of the substrate. The direction of the stream iscontrolled electronically in causing the droplets to print the desiredimage or information on the substrate surface without requiring contactbetween the printing device and the surface to which the ink is applied.Ink jet printing may be applied to a variety of substrates including,but not limited to, metals, glass, synthetic resins, plastics, rubber,paper, and the like. Objects, comprising substrates to which ink jetprinting is well suited, include, but are not limited to, containers forconsumer products, currency, draft checks, envelopes, letterhead,identification cards, bank cards (debit, credit, and the like),identification strips (e.g., comprising barcodes), and the like.

Fluorescent inks have been developed for printing a “security mark” onarticles such that the mark is invisible to the unaided eye, but thatcan be detected as fluorescence upon excitation with an activating lightof a suitable excitation wavelength spectrum. More particularly,security marks are applied to articles in efforts to prevent forgery,theft, and fraud; wherein such articles are known to include, but arenot limited to, identification cards, passports, currency, checks,securities, and other types of commercial paper. The security mark maybe in the form of a barcode which encodes information, or may comprise arecognizable pattern for identification and verification purposes. Priorart fluorescent inks are known in the art to include: a fluorescentcolorant, a solvent, and a binder resin; an organic laser dye that isexcited by a wavelength spectrum in the near infrared range andfluoresces in an infrared range; a phenoxazine derivative dyefluorescing in the near infrared range; a rare earth metal, an organicink carrier, and may further comprise a chelating agent; and a nearinfrared fluorophore and a water-dissipatable polyester. Generally, suchfluorescent inks comprising fluorescent dyes or pigments, present withseveral disadvantages. For example, there is a relatively narrow marginbetween the amount of a fluorescent dye which is sufficient to give goodfluorescent color intensity, and an amount wherein the dye moleculesbegin to aggregate and thus reduce the amount of fluorescence byquenching. A limitation in intensity can also limit the density ofinformation encoded on a security mark (such as a barcode) by afluorescent ink composition.

Thus, there exists a need for fluorescent ink compositions suitable forprinting on substrates, wherein (a) the fluorescent component of thefluorescent ink composition is water-soluble; (b) the fluorescentcomponent is excited by a wavelength spectrum comprising UV light, andpreferably in a spectral range of from about 300 nanometers (nm) toabout 400 nm, and emits an narrow emission peak in a wavelength spectrumprimarily in the visible range, and preferably in a spectral range offrom about 410 nm to about 750 nm; (c) a plurality of fluorescent inkcompositions (each containing a fluorescent component that can bedetectably distinguished (e.g., by fluorescent color and/or intensity)from that of other fluorescent ink compositions of the plurality offluorescent ink compositions) may be utilized for multicolorfluorescence by excitation with a single wavelength spectrum of lightresulting in simultaneous detection of fluorescence of high quantumyield and with discrete peak emission spectra; (d) a fluorescentcomponent that resists photobleaching (and therefore can be used forsignal integration); and (e) a fluorescent component that is notsusceptible to quenching.

SUMMARY OF THE INVENTION

Provided are fluorescent ink compositions comprising a fluorescentcomponent that comprises functionalized fluorescent nanocrystals (e.g.,a single type for print of a single fluorescent color, or a plurality oftypes for print of multicolor), an ink carrier, and a binder; and mayfurther comprise other components such as one or more of a biocide, asurfactant, a defoamer, and the like. One or more fluorescent inkcompositions are printed onto the surface of a substrate using a printerdevice. Following excitation of the printed surface with an appropriateexcitation wavelength spectrum, each of the one or more fluorescent inkcompositions printed on the surface will emit fluorescence of highquantum yield and with discrete peak emission. The fluorescent inkcomposition may be varied with respect to the intensity of fluorescenceemission. Variables such as differences in intensity and in fluorescentcolor can enable an increase in the information that can be stored in asecurity mark, as well as increase the possible number and complexity ofa security mark so as to prevent forgery. For example, the fluorescentink composition can be varied by controlling the amount and type offunctionalized fluorescent nanocrystals in the method of preparing thefluorescent ink composition. Thus, the fluorescent properties of thefluorescent ink composition, such as intensity and color, are sensitiveto the functionalized fluorescent nanocrystals made apart thereof. Aresultant advantage of the fluorescent ink compositions of the presentinvention is that they may be produced to have a greater degree offluorescence (intensity) than previously known fluorescent jetcompositions.

An additional advantage of the use of the fluorescent ink compositionsaccording to the present invention relates to the use of a plurality ofthe fluorescent ink compositions in printing a desired pattern onto asurface. Each fluorescent ink composition may be comprised of a type offunctionalized fluorescent nanocrystals capable of fluorescing aspecific color. Thus, the plurality of fluorescent ink compositionsmaybe used to print a desired pattern which be excited to emitmulticolor fluorescence which may be visible simultaneously ingenerating a specific, identifiable code (based on the emission spectrawhich can comprise both color and intensity) that can be used forpurposes of verification or identification. For example, a plurality offluorescent ink compositions may comprise a first ink compositioncapable of fluorescing red, a second ink composition capable offluorescing blue, a third ink composition capable of fluorescing yellow,and a fourth composition capable of fluorescing green. This exemplarycombination of a fluorescent ink compositions may be used to printcurrency with a security mark comprising a code (based on the number, ornumber and intensity, of colors) for verification purposes. Further, ina method of producing the fluorescent ink composition according to thepresent invention, by controlling the proportion of the components,precise control may be achieved with respect to the basic fluorescentproperties of the resultant fluorescent ink composition.

The above and other objects, features, and advantages of the presentinvention will be apparent in the following Detailed Description of theInvention.

DETAILED DESCRIPTION OF THE INVENTION Definitions

By the term “functionalized fluorescent nanocrystals” is meant, forpurposes of the specification and claims to refer to nanocrystalscomprising semiconductor nanocrystals or doped metal oxide nanocrystals,wherein the nanocrystals are operably bound to, and functionalized bythe addition of, a plurality of molecules which provide thefunctionalized fluorescent nanocrystals with reactive functionalities toenable the fluorescent nanocrystals to be soluble in the aqueous-basedink carrier of the fluorescent ink composition; wherein the plurality ofmolecules may be selected from the group consisting of carboxylic acid,an amino acid, a diaminocarboxylic acid, a monoaminocarboxylic acid, anda combination thereof. Further, the reactive functionalities may aid thefunctionalized nanocrystals to become bound to one or more of the binderof the fluorescent ink composition, or the substrate onto which they areapplied. A preferred type of molecules to functionalized the fluorescentnanocrystals may be used to the exclusion of molecules other than thepreferred type of molecules. The functionalized nanocrystals aresufficiently soluble in an aqueous-based environment provided by an inkcarrier in forming an aqueous-based fluorescent ink composition.

By the term “semiconductor nanocrystals” is meant, for purposes of thespecification and claims to refer to quantum dots (crystallinesemiconductors) comprised of a core comprised of at least one of a GroupII-VI semiconductor material (of which ZnS, and CdSe are illustrativeexamples), or a Group III-V semiconductor material (of which GaAs is anillustrative example), a Group IV semiconductor material, or acombination thereof. In a preferred embodiment, the core of the quantumdots may be passivated with an semiconductor overlayering (“shell”)uniformly deposited thereon. For example, a Group II-VI semiconductorcore may be passivated with a Group II-VI semiconductor shell (e.g., aZnS or CdSe core may be passivated with a shell comprised of YZ whereinY is Cd or Zn, and Z is S, or Se). As known to those skilled in the art,the size of the semiconductor core correlates with the spectral range ofemission. Table 1 is an illustrative example for CdSe.

TABLE 1 Color Size Range (nm) Peak Emission Range blue 2.5 to 2.68 476to 486 green 2.8 to 3.4  500 to 530 yellow 3.58 to 4.26  536 to 564orange 4.9 to 6.1  590 to 620 red 8.6 to 10.2 644 to 654

In a preferred embodiment, the semiconductor nanocrystals are producedusing a continuous flow process and system disclosed in copending U.S.application Ser. No. 09/468,418 (the disclosure of which is hereinincorporated by reference), and have a particle size that varies by lessthan +/−4% in the average particle size. In a preferred embodiment, thesemiconductor nanocrystals comprise a monodisperse population having anaverage particle size (as measure by diameter) in the range ofapproximately 1 nanometer (nm) to approximately 20 nm.

By the term “doped metal oxide nanocrystals” is meant, for purposes ofthe specification and claims to refer to nanocrystals comprised of: ametal oxide, and a dopant comprised of one or more rare earth elements.For example, suitable metal oxides include, but are not limited to,yttrium oxide (Y₂O₃), zirconium oxide (ZrO₂), zinc oxide (ZnO), copperoxide (CuO or CU₂O), gadolinium oxide (Gd₂O₃), praseodymium oxide(Pr₂O₃), lanthanum oxide (La₂O₃), and alloys thereof. The rare earthelement comprises an element selected from the Lanthanide series andincludes, but is not limited to, europium (Eu), cerium (Ce), neodymium(Nd), samarium (Sm), terbium (Tb), gadolinium (Gd), holmium (Ho),thulium (Tm), an oxide thereof, and a combination thereof. As known tothose skilled in the art, depending on the dopant, an energized dopedmetal oxide nanocrystal is capable of emitting light of a particularcolor. Thus, the nature of the rare earth or rare earths are selected inconsequence to the color sought to be imparted (emitted) by a dopedmetal oxide nanocrystal used in the fluorescent ink compositionaccording to the present invention. A given rare earth or rare earthcombination has a given color, thereby permitting the provision of dopedmetal oxide nanocrystals, each of which may emit (with a narrow emissionpeak) a color over an entire range of colors by adjusting the nature ofthe dopant, the concentration of the dopant, or a combination thereof.For example, the emission color and brightness (e.g. intensity) of adoped metal oxide nanocrystal comprising Y₂O₃:Eu may depend on theconcentration of Eu; e.g., emission color may shift from yellow to redwith increasing Eu concentration. For purposes of illustration only,representative colors which may be provided are listed in Table 2.

TABLE 2 Fluorescent Color Dopant blue thulium blue cerium yellow-greenterbium green holmium green erbium red europium reddish orange samariumorange neodymium yellow dysprosium white praseodymium orange-yelloweuropium + terbium orange-red europium + samarium

Methods for making doped metal oxide nanocrystals are known to include,but are not limited to a sol-gel process (see, e.g., U.S. Pat. No.5,637,258), and an organometallic reaction. As will be apparent to oneskilled in the art, the dopant (e.g., one or more rare earth elements)are incorporated into the doped metal oxide nanocrystal in a sufficientamount to permit the doped metal oxide nanocrystal to be put topractical use in fluorescence detection as described herein in moredetail. An insufficient amount comprises either too little dopant whichwould fail to emit sufficient detectable fluorescence, or too muchdopant which would cause reduced fluorescence due to concentrationquenching. In a preferred embodiment, the amount of dopant in a dopedmetal oxide nanocrystal is a molar amount in the doped metal oxidenanocrystal selected in the range of from about 0.1% to about 25%. Dopedmetal oxide nanocrystals may can be excited with a single excitationlight source resulting in a detectable fluorescence emission of highquantum yield (e.g., a single quantum dot having at a fluorescenceintensity that may be a log or more greater than that a molecule of aconventional fluorescent dye) and with a discrete fluorescence peak.Typically, they have a substantially uniform size of less than 200Angstroms, and preferably have a substantially uniform size in the rangeof sizes of from about 1 nm to about 5 nm, or less than 1 nm. In apreferred embodiment, the doped metal oxide nanocrystals are comprisedof metal oxides doped with one or more rare earth elements, wherein thedopant comprising the rare earth element is capable of being excited(e.g., with ultra-violet light) to produce a narrow spectrum offluorescence emission. In another preferred embodiment, the doped metaloxide has both fluorescent properties (when excited with an excitationlight source) and magnetic properties. Thus, in one embodiment, afluorescent ink composition may comprise functionalized fluorescentnanocrystals comprising doped metal oxide nanocrystals which aremagnetic.

By the term “operably bound” is meant, for purposes of the specificationand claims to refer to fusion or bond or an association of sufficientstability to withstand conditions encountered in a method of detection,between a combination of different molecules such as, but not limitedto, between functionalized fluorescent nanocrystals and the binder, andbetween a fluorescent nanocrystal and the molecules with which it isfunctionalized (e.g., carboxylic acid, diaminocarboxylic acid, or amonoaminocarboxylic acid), and a combination thereof. As known to thoseskilled in the art, the bond may comprise one or more of covalent,ionic, hydrogen, van der Waals, and the like. As known to those skilledin the art, and as will be more apparent by the following embodiments,there are several methods and compositions in which two or moremolecules may be operably bound utilizing reactive functionalities.Reactive functionalities include, but are not limited to, free chemicalgroups (e.g., thiol, or carboxyl, hydroxyl, amino, amine, sulfo, etc.),and reactive chemical groups (reactive with free chemical groups).

The term “suitable conditions” is used herein, for purposes of thespecification and claims, and with reference to a process of reactingtwo components (e.g., functionalized fluorescent nanocrystals and abinder), to mean those conditions under which the components may becomeoperably bound to each other. As known to those skilled in the art, suchconditions may include one or more of: a pH range of from about 3 toabout 9, ionic strengths such as that ranging from distilled water toabout 1 molar sodium chloride, and a temperature in the range of fromabout 4° C. to about 45° C.; and may further include a time sufficientfor binding to occur (e.g., in a range of from about 10 minutes to about2 hours).

By the term “diaminocarboxylic acid” is meant, for purposes of thespecification and claims to refer to an amino acid that has two freeamine groups. The amino acid may be a naturally occurring amino acid, asynthetic amino acid, a modified amino acid, an amino acid derivative,and an amino acid precursor (e.g., citrulline and ornithine areintermediates in the synthesis of arginine). In a preferred embodiment,the diaminocarboxylic acid contains neutral (uncharged) polar functionalgroups which can hydrogen bond with water, thereby making thediaminocarboxylic acid (and the quantum dot to which it is made a partof) relatively more soluble in aqueous solutions containing water thanthose with nonpolar functional groups. Exemplary diaminocarboxylic acidsinclude, but are not limited to, lysine, asparagine; glutamine,arginine,. citrulline, ornithine, 5-hydroxylysine, djenkolic acid,β-cyanoalanine, and synthetic diaminocarboxylic acids such as3,4-diaminobenzoic acid, 2,3-diaminopropionic acid,2,4-diaminobutyricacid, 2,5-diaminopentanoic acid, and2,6-diaminopimelic acid.

By the term “amino acid” is meant, for purposes of the specification andclaims to refer to a molecule that has at least one free amine group andat least one free carboxyl group. The amino acid may have more than onefree amine group, or more than one free carboxyl group, or may furthercomprise one or more free chemical reactive groups other than an amineor a carboxyl group (e.g., a hydroxyl, a sulfhydryl, a seleno group,etc.). The amino acid may be a naturally occurring amino acid, asynthetic amino acid, a modified amino acid, an amino acid derivative,and an amino acid precursor. The amino acid may further be selected fromthe group consisting of a monoaminocarboxylic acid, and adiaminocarboxylic acid. In a preferred embodiment, themonoaminocarboxylic acid contains one or more neutral (uncharged) polarfunctional groups which can hydrogen bond with water, thereby making themonoaminocarboxylic acid (and the quantum dot to which it is made a partof) relatively more soluble in aqueous solutions containing water thanthose with non-polar functional groups. Exemplary monoaminocarboxylicacids include, but are not limited to, glycine, serine, threonine,cysteine, β-alanine, homoserine, γ-aminobutyric acid, and homocysteine.Other preferred amino acids include sulfur containing amino acids (e.g.,cysteine, homocysteine, s-allyl-L-cysteine sulfoxide, taurine,penicillamine, and the like); and seleno containing amino acids (e.g.,selenocysteine, selenohomocysteine, Semethylselenocysteine,selenocystine, selenohomocystine, and the like).

By the term “carboxylic acid” is meant, for purposes of thespecification and claims to refer to a compound having the formula HS(CH₂)_(n)X, wherein X is a carboxylate (carboxylic moiety). “n” is anumber in the range of from 1 to about 20, and preferably greater than4. In a preferred embodiment, the thiol group of the carboxylic acid canbe used as a reactive functionality for the carboxylic acid to becomeoperably bound to the nanocrystal, depending on the composition of thenanocrystal (e.g., to Cd, Zn and the like). Additionally, the carboxylicmoiety of the carboxylic acid imparts water solubility to thenanocrystals. Exemplary carboxylic acids may include, but are notlimited to, mercaptocarboxylic acid, or mercaptofunctionalized amines(e.g., aminoethanethiol-HCl, or 1-amino-2-methyl-2-propanethiol-HCl).

The present invention provides a fluorescent ink composition, suitablefor printing a desired pattern on a substrate, comprising functionalizedfluorescent nanocrystals, and an ink carrier. Also provided is afluorescent ink composition comprising functionalized fluorescentnanocrystals, an ink carrier, and a binder. Also provided is afluorescent ink composition comprising functionalized fluorescentnanocrystals, an ink carrier, a binder, and one or more additionalcomponents suitable for the function of an ink composition. In apreferred embodiment, the functionalized fluorescent nanocrystals areexcitable by ultra-violet light, and fluoresce at a spectral wavelengthhaving a peak emission in the range of from about 410 nm to about 900nm. In a more preferred embodiment, the fluorescence is in the range ofvisible light.

FUNCTIONALIZED FLUORESCENT NANOCRYSTALS

As disclosed in detail in U.S. Pat. No. 6,114,038 (the disclosure ofwhich is herein incorporated by reference), fluorescent nanocrystalscomprise nanocrystals which have been functionalized by the addition ofa plurality of molecules; and preferably, the molecules are selectedfrom an amino acid, a carboxylic acid, and a combination thereof. Forexample, the nanocrystals may comprise semiconductor nanocrystals thathave a core selected from the group consisting of CdSe, CdS, and CdTe(collectively referred to as “Cdx”), and may further comprise apassivating shell comprised of YZ wherein Y is Cd or Zn, and Z is S, orSe. Typically, CdX core/YZ shell quantum dots are overcoated withtrialkylphosphine oxide, with the alkyl groups most commonly used beingbutyl and octyl. In one preferred embodiment, the CdX core/YZ shellquantum dots, are treated with a large excess of mercaptocarboxylic acidin exchanging the trialkylphosphine oxide coat with a coat comprising aplurality of carboxylic acid molecules. For example, (CdSe)ZnSnanocrystals were prepared in a pyridine solution. The pyridineovercoating of the (CdX) core/YZ shell nanocrystals were exchanged witha carboxylic acid comprising mercaptocarboxylic acid. Exchange of thecoating group is accomplished by treating the water-insoluble,pyridine-capped quantum dots with a large excess of neatmercaptocarboxylic acid. To accomplish this, the pyridine-capped(CdSe)ZnS quantum dots were precipitated with hexanes, and then isolatedby centrifugation. The residue was dissolved in neat mercaptoaceticacid, with a few drops of pyridine added, if necessary, to form atransparent solution. Chloroform was added to precipitate thenanocrystals and wash away excess thiol. The nanocrystals were isolatedby centrifugation, washed once more with chloroform, and then washedwith hexanes. The residue was briefly dried with a stream of argon. Theresultant nanocrystals, coated with molecules of carboxylic acid, werethen soluble in water or other aqueous solutions. The nanocrystals, inan aqueous solution, were centrifuged once more, filtered through a 0.2μm filter, degassed with argon, and stored in an amber vial. Thenanocrystals may then be further functionalized by an amino acidcomprising a diaminocarboxylic acid. The diaminocarboxylic acidmolecules were operably bound to the carboxylic acid molecules of thenanocrystals by using commercially available crosslinking agents andmethods known to those skilled in the art. For example, the carboxylicacid-coated nanocrystals were dissolved in an aqueous buffer system (pHof about 7). To the nanocrystals was added EDC(1-ethyl-3-[3-dimethylaminopropyl]carbodiimide) and sulfoNHS(sulfo-N-hydroxysuccinimide) in 500-1000 times excess. The resultingsolution was stirred at room temperature for 30 minutes. Mercaptoethanolwas added to neutralize unreacted EDC at 20 mM concentration and stirredfor 15 minutes. The entire solution was then added drop-wise, withstirring, to a solution of a diaminocarboxylic acid comprising lysine(large excess) in the same buffer; and the mixture was stirred for 2hours at room temperature. Ethanolamine (30 mM) was added to quench thereaction; and the mixture was stirred for 30 minutes at room temperatureor left overnight at 4° C. The solution was centrifuged to remove anyprecipitated solids, and then ultrafiltered through a 30 kD MWcentrifugal filter. The resultant concentrated, fluorescent nanocrystalscan be solubilized in an aqueous solution of choice. Once solubilized,the resulting solution can be stored in an amber vial under an inert gasto prevent flocculation. The fluorescent nanocrystals may be operablybound to a successive layer of amino acid molecules by, for example,repeating the procedure and reaction using EDC and sulfoNHS with theamino acid molecules comprising the successive layer.

Similarly, a nanocrystal comprising a doped metal oxide nanocrystal maybe operably bound to a plurality of molecules (e.g., a carboxylic acid,and amino acid, or a combination thereof) using methods known in theart. For example, the plurality of molecules having reactivefunctionalities comprising free carboxyl groups can be chemi-sorbed,adsorbed or otherwise permanently added to the metal oxide portion ofthe nanocrystal. For example, the metal oxide nanocrystals are suspendedin an aqueous solution of an amino acid comprising homocysteine having apH of about 3.5 for about an hour. The reaction is then stopped byadjusting the pH to neutral, and dialyzing out the aqueous solution.

As an alternative, fluorescent nanocrystals functionalized with aplurality of a sulfur containing amino acid or a seleno containing aminoacid. In a preferred embodiment, the fluorescent nanocrystals werefunctionalized with homocysteine molecules. Nanocrystals (e.g.,(CdSe)ZnS) coated with an organic layer (e.g., mercaptoacetic acid) weretreated with a molar excess of homocysteine in replacing the organiclayer with a coating comprising a plurality of homocysteine molecules.The approximate number of surface Zn sites on the specific size ofnanocrystals utilized was calculated. At least a 5 times molar excess ofhomocysteine (as compared to the number of surface Zn sites) was addedto the nanocrystals, as per the following formula. Gramshomocysteine=5(number of Zn surface sites) (volume of solutioncontaining the nanocrystals) (concentration of nanocrystals in solution)(135.2).

The mixture was stirred to dissolve the homocysteine, and then stored at4° C. for 24 hours. The resultant solution was then centrifuged toremove any precipitate, and the supernatant was transferred to acentrifugal filter for the appropriate volume of supernatant(preferably, with a molecular weight cutoff of about 10 kD or below toretain the fluorescent nanocrystals coated with homocysteine). Aftercentrifugation, and when the desired minimum volume is reached, thefluorescent nanocrystals were then rediluted in the appropriate aqueoussolution (e.g., HEPES buffer) to a volume in which the original mass ofhomocysteine had been dissolved. The steps of filtering and redilutionof the fluorescent nanocrystals in solution may be repeated to improvepurity. The resultant fluorescent nanocrystals comprisinghomocysteine-coated nanocrystals may then be degassed by bubbling withan inert gas, and then stored at 4° C. in an amber bottle.

The functionalized fluorescent nanocrystals used in the fluorescent inkcomposition according to the present invention are (a) functionalized tobe water soluble and to enhance stability in water or a water-basedsolution; (b) a class of nanocrystals that may be excited with a singleexcitation light source resulting in detectable fluorescence emissionsof high quantum yield and with discrete fluorescence peaks; (c)functionalized so as to have reactive functionalities, such as may beused to operably bind to a binder, if desired; and (d) resistant tophotobleaching and fading. “Water-soluble” is used herein to meansufficiently soluble or dispersible in water or water-based solutions.Preferably the functionalized fluorescent nanocrystals used in afluorescent ink composition comprise a substantially uniform size ofless than 100 Angstroms, and preferably have a substantially uniformsize in the range of from about 2 nm to about 10 nm (diameter). Asapparent to one skilled in the art, the amount of the functionalizedfluorescent nanocrystals contained within the fluorescent inkcomposition according to, the present invention will depend on factorssuch as the desired intensity of the emitted fluorescence, the nature ofthe fluorescent nanocrystals comprising the functionalized fluorescentnanocrystals, the desired viscosity of the fluorescent ink composition,the nature of the substrate onto which is printed the fluorescent inkcomposition, and the nature of the printer device (e.g., ink jetprinter) for which the fluorescent ink composition is intended. In apreferred embodiment, the amount of functionalized fluorescentnanocrystals is in the range of from about 0.0005% to about 5%, based onthe total weight of the fluorescent ink composition.

Ink Carrier

Any suitable aqueous-based ink carrier may be used to prepare thefluorescent ink composition according to the present invention. Suitableaqueous-based ink carriers should provide sufficient solubility for thevarious components of the fluorescent ink composition (e.g.,functionalized fluorescent nanocrystals and binder). A suitableaqueous-based ink carrier comprises water (preferably deionized water),or a mixture of water and at least one water-soluble organic solvent. Asapparent to one skilled in the art, the selection of the ink carrierwill depend on factors such as the desired drying time of thefluorescent ink composition, the desired surface tension and viscosityof the fluorescent ink composition, the nature of the substrate ontowhich is printed the fluorescent ink composition, and the nature of theprinter device (e.g., ink jet printer) for which the fluorescent inkcomposition is intended. Similarly, depending on such factors, theamount of the ink carrier in the fluorescent ink composition accordingto the present invention may vary. In a preferred embodiment, the amountof ink carrier is in the range of from about 75% to about 99.5%, basedon the total weight of the fluorescent ink composition.

In the case of an aqueous-based ink carrier comprising a mixture ofwater and at least one water-soluble organic solvent, the amount ofwater is sufficient for solubilization of the functionalized fluorescentnanocrystals therein. In a preferred embodiment, the aqueous-based inkcarrier contains from about 40% to about 95% water (based on totalweight of the aqueous-based ink carrier), and more preferably, fromabout 60% to about 95% water; with the balance being the one or morewater-soluble organic solvents. It is further preferred that thewater-soluble organic solvent not adversely affect the peak fluorescentemission wavelength or the intensity of the fluorescent emission ofexcited functionalized fluorescent nanocrystals in the fluorescent inkcomposition. Water-soluble solvents are known to those skilled in theart to include, but are not limited to, citrus solvents (e.g., citruspeel oils, terpine, terpinene, dipentene, and the like), a polyhydricalcohol (e.g., ethylene glycol, propylene glycol, butylene glycol,triethylene glycol, thiodiglycol, diethylene glycol, hexylene glycol,and the like), an alcohol (e.g.,.methanol, ethanol, propanol, butanol),a polyalkyl glycol (e.g., polyethylene glycol), alkyl ethers of apolyhydric alcohol, ketones or ketoalcohols, sulfur containing compounds(e.g., dimethyl sulfoxide, tetramethyl sulfone), and esters (e.g.,ethylene carbonate, propylene carbonate).

Binder

The fluorescent ink composition may optionally comprise a binder. Thebinder serves to immobilize, or increase the adhesion of, thefluorescent ink composition to the substrate on which it is printed. Thebinder is soluble or dispersible in the ink carrier of the fluorescentink composition, and preferably should not adversely affect the peakfluorescent emission wavelength or the intensity of the fluorescentemission of excited functionalized fluorescent nanocrystals in thefluorescent ink composition. A water-soluble or water-dispersible binderis known to those skilled in the art to include, but is not limited to,one or more of: an acrylic polymer, starch, a polyvinyl alcohol,glycerol, polyethylene glycol, carboxymethylcellulose, hydroxyethylcellulose, polysodium acrylate, polysodium methacrylate, styrene-acryliccopolymers, styrene-maleic copolymers, pectinic acid, and the like. Asapparent to one skilled in the art, the selection and amount of thebinder will depend on factors such as the desired surface tension andviscosity of the fluorescent ink composition, the desired waterdispersibility, the nature of the substrate onto which is printed thefluorescent ink composition, and the nature of the printer device (e.g.,ink jet printer) for which the fluorescent ink composition is intended.In a preferred embodiment, the amount of binder is in the range of fromabout 1% to about 25%, based on the total weight of the fluorescent inkcomposition. In a preferred embodiment in which the fluorescent inkcomposition is used in an ink jet printer, a suitable amount of binderis added to the fluorescent ink composition so as to comprise aviscosity suitable for ink jet printing; i.e., within the range of fromabout 1 to 20 centiposes (cps), and more preferably, 1.5 to 8 cps.

In a most preferred embodiment, each molecule of binder comprises one ormore reactive functionalities that is reactive with (e.g., can be usedto operably bind to) one or more reactive functionalities offunctionalized fluorescent nanocrystals in forming the fluorescent inkcomposition according to the present invention. Operably binding thebinder to the functionalized fluorescent nanocrystals can result inproduction of a durable cross-linked fluorescent ink composition thatmay improve adhesion and surface tension of the fluorescent inkcomposition to the surface of the substrate onto which it is printed; aswell as improving the stability of the fluorescent ink composition instorage and/or printing operation conditions. For example, a molecule ofbinder comprises one or more free chemical groups, and thefunctionalized fluorescent nanocrystals each comprise multiple moleculesof reactive functionality comprising reactive chemical groups which arecapable of becoming operably bound (e.g., ionically) to the reactivefunctionalities of the binder under suitable conditions so that formedis a fluorescent ink composition comprised of functionalized fluorescentnanocrystals cross-linked with binder. In continuing with anillustrative example, a molecule of binder comprises reactivefunctionalities comprising amino groups, and the functionalizedfluorescent nanocrystals comprise reactive functionalities comprisingamino-reactive groups (e.g., carboxyl or amine). In another illustrativeexample, the binder comprises reactive functionalities comprisingcarboxyl groups and the functionalized fluorescent nanocrystals comprisereactive functionalities comprising carboxyl-reactive groups (e.g.,amine). Binders having at least one free carboxyl group may include, butare not limited to, carboxylated cellulose derivatives (e.g.,carboxyethyl cellulose, carboxypropyl cellulose, carboxymethylcellulose, hydroxylalkyl celluloses, alkyl celluloses, hydroxypropylmethylcellulose), alginic acid, pectinic acid, esterified starches,polymeric carboxylic acids (e.g., polyacrylic acids, polymethacrylicacids, polymaleic acids), polymeric sulfonic acids, and polycarboxylatedvinyl polymers. A preferred binder comprises carboxymethyl cellulose.Binders having at least one amine group may include, but are not limitedto, polyethylene glycol end-capped with amine, caseinate, soybeanprotein, gelatin, polyvinyl amines, polyallylamines,amine-functionalized lignin, amine-functionalized acrylic resins (e.g.,dimethyl amino ethyl methacrylate), and amine-functionalized cellulosederivatives (e.g., aminoethylcellulose, aminopropylcellulose). Thebinder of the fluorescent ink composition according to the presentinvention may comprise one type or a plurality of types of binders.

In a preferred embodiment, the functionalized fluorescent nanocrystals,ink carrier, and binder comprise at least about 90% by weight of thefluorescent ink composition.

Additional Components

The fluorescent ink composition may comprise an additional componentselected from the group consisting of a biocide, a defoamer, asurfactant, a,corrosion inhibitor, and a combination thereof.Surfactants may be added to improve the wettability of the fluorescentink composition to the surface of the substrate onto which it isprinted. Surfactants may include, but are not limited to, ethoxylatedtetramethyl decynediol, alkylbenzene sulfonates, alkylnaphthylsulfonates, alcohol sulfates, perfluorinated carboxylic acids,alkylesters of polyethylene glycol, fatty acid esters of glycol,alkylamines, amphoteric surfactants, and a combination thereof. In apreferred embodiment in which the fluorescent ink composition comprisesa surfactant as a component, the surfactant may be present in thefluorescent ink composition in an amount of from about 0.01% to about1.5% by weight of the fluorescent ink composition. A defoamer preventsfoaming of the fluorescent ink composition during its preparation, aswell as during the printing operation. Any suitable defoamer known tothose skilled in the art may be used as an additional component in thefluorescent ink composition according to the present invention; and morepreferably, the defoamer is soluble or dispersible in the ink carrier ofthe fluorescent ink composition. A defoamer may comprise one or more ofa mixture of tetramethyldecynediol and propylene glycol, a siliconedefoamer, an acetylenic defoamer, and the like. In a preferredembodiment in which the fluorescent ink composition comprises a defoameras a component, the defoamer may be present in the fluorescent inkcomposition in an amount of from about 0.5% to about 1.2% by weight ofthe fluorescent ink composition. A biocide is one or more agents toprevent growth of bacteria, mold, or fungus in the fluorescent inkcomposition. Any suitable biocide known to those skilled in the art maybe used as an additional component in the fluorescent ink compositionaccording to the present invention; and more preferably, the biocide issoluble or dispersible in the ink carrier of the fluorescent inkcomposition. A suitable biocide may include, but is not limited to, oneor more of benzoate salts, sorbate salts, methyl p-hydroxybenzoate,6-acetoxy-2,2-dimethyl-1,3-dioxane, 1,2-benzisothiazolin-3-one, and thelike. In a preferred embodiment in which the fluorescent ink compositioncomprises a biocide as a component, the biocide may be present in thefluorescent ink composition in an amount of from about 0.02% to about0.5% by weight of the fluorescent ink composition. A corrosion inhibitormay be added to the fluorescent ink composition to inhibit or reducecorrosion, such as of the metal parts of the printer device (e.g.,nozzles or orifices). Any suitable corrosion inhibitor known to thoseskilled in the art may be used as an additional component in thefluorescent ink composition according to the present invention; and morepreferably, the corrosion inhibitor is soluble or dispersible in the inkcarrier of the fluorescent ink composition. A suitable corrosioninhibitor may include, but is not limited to, one or more of the1H-benzotriazoles. In a preferred embodiment in which the fluorescentink composition comprises a corrosion inhibitor as a component, thecorrosion inhibitor may be present in the fluorescent ink composition inan amount of from about 0.01% to about 0.5% by weight of the fluorescentink composition.

Method of Manufacture

As appreciated by those skilled in the art of inks, the fluorescent inkcompositions according to the present invention may be prepared in anynumber of ways. However, a preferred method of formulating thefluorescent ink compositions comprises mixing the functionalizedfluorescent nanocrystals into the aqueous-based ink carrier comprisingwater or a water-based solution, and then added to and mixed therein isa binder. For fluorescent ink compositions that further comprise one ormore additional components (e.g., surfactant, biocide, corrosioninhibitor), the one or more additional components may then be added toand mixed in the fluorescent ink composition. The final mixture may bestirred until a blended, consistent composition is obtained. Thefluorescent ink composition maybe filtered, if desired, e.g., to removeany impurities.

Printing Devices

As apparent to one skilled in the art of printing, the printing deviceutilized will depend on the printing method in which the fluorescent inkcompositions according to the present invention are applied. In oneembodiment, each of a plurality of the fluorescent ink compositions isplaced in a separate well of a printing device containing multiple wellsfor holding ink or ink cartridges, in loading the printing device withfluorescent ink compositions that may be used in combination to print animage (e.g., security mark or one or more patterns) that is capable offluorescing in multiple colors when exposed to a single excitation lightsource suitable for exciting the functionalized fluorescent nanocrystalsof the fluorescent ink compositions. The fluorescent inks are printedonto a substrate by the printing device in ratios appropriate to yieldthe desired colors and to form the desired image. In a preferredembodiment, the printing device is an ink jet printer.

Printing Methods

The fluorescent ink compositions according to the present invention maybe used in many printing methods such as ink jet printing, screenprinting, gravure printing, letterpress printing, offset printing,relief printing, intaglio printing and the like. As apparent to oneskilled in the art, the viscosity of the fluorescent ink compositionswill vary depending on the printing method, the printer device, and thesubstrate onto which the fluorescent ink compositions are printed. Forexample, a preferred viscosity for ink jet printing may range from about1.5 to about 15 cps; and more preferably, from about 3 to about 6 cps.In contrast, a preferred viscosity for screen printing may range fromabout 100 to about 400 cps; and more preferably, from about 200 to about300 cps. Printed images, generated using the printing method andfluorescent ink compositions according to the present invention, arefluorescent when exposed to an excitation light source (e.g., UV light);and thus become visible or detectable as a color (or colors when morethan one fluorescent ink composition is used in the printing method) inthe range of from about 410 nm to about 900 nm.

Substrates

The fluorescent ink compositions according to the present invention maybe used to print on a number of suitable substrates which may include,but are not limited to, conventional papers.(computer paper, currencypaper, bond paper, copying paper, image paper), glass, rubber, vinyl,plastics, fabrics, films, inorganic substrates (e.g., metals and woods),and the like. In a preferred embodiment, the substrate comprises aporous or absorbent substrate, such as paper.

Methods for Identification of an Object

As apparent to those skilled in the art from the descriptions herein,one or more of the fluorescent ink compositions of the present inventionmay be used in a method of providing an object with a security markingfor purposes of identification (which may further encompassverification). In one embodiment, a method for identifying an objectcomprising the steps of:

(a) applying a security mark to the object using a fluorescent inkcomposition comprising functionalized fluorescent nanocrystals, anaqueous-based ink carrier comprising water or a water-based solution,and a binder;

(b) exposing the object to an excitation light source having awavelength spectrum in a range of from about 300 nm to about 400 nm;

(c) visualizing the security mark by detecting a peak fluorescentemission in a spectral range of from about 410 nm to about 750 nm; and

(d) identifying the object in response to visualizing the security mark.

In another embodiment, a plurality of fluorescent ink compositions areused to apply a security mark to the object, wherein each of theplurality of fluorescent ink compositions is capable of fluorescing aspecific (e.g., different than that emitted by any other of thefluorescent ink compositions in the plurality of fluorescent inkcompositions) color in providing a security mark which fluoresces inmulticolor when visualized; and identification is based on thevisualization of the security mark fluorescing in multicolor.

As appreciated by those skilled in the art, the exact components, theproportion of the components, and properties of the components, desiredfor an ink composition will depend on the application, printing method,and substrate. Therefore, routine experimentation may be required todetermine the components and the proportion of the components which areoptimum in forming a fluorescent ink composition for a give applicationand with the desired properties.

The following examples are provided to further illustrate the presentinvention, and are not to be construed as limiting the invention in anymanner.

EXAMPLE 1

For purposes of illustration, listed in Table 3 are various examples ofa fluorescent ink composition according to the present invention,wherein the fluorescent ink composition comprised functionalizedfluorescent nanocrystals, and an ink carrier. The components are listedby their weight percent of the composition; wherein the functionalizedfluorescent nanocrystals (e.g., homocysteine-coated fluorescentnanocrystals) are abbreviated as “FNC”; and the ink carrier (“IC”)comprised water (H₂O) and at least one water soluble organic solvent(“WSO”). The water soluble organic solvents used for purposes ofillustration included dimethyl sulfoxide (“DMSO”) and polyethyleneglycol (“PEG”). The resultant fluorescent ink compositions were printedon a white plain paper substrate (a stock of standard copy paper lackingsubstantial background fluorescence), and allowed to dry. The print wasinvisible to the unaided eye. The paper substrate was then exposed to UVlight, and the print appeared the appropriate color due to thefluorescence of the fluorescent ink composition used.

TABLE 3 FNC IC <1% H₂O > 94%; DMSO 5% <1% H₂O > 59%; DMSO 40% <1% H₂O >94%; PEG 5% <1% H₂O > 59%; PEG 40%

EXAMPLE 2

For purposes of illustration, listed in Table 4 are various examples ofa fluorescent ink composition according to the present invention,wherein the fluorescent ink composition comprised functionalizedfluorescent nanocrystals (“FNC”), an ink carrier. (“IC”), and a binder.The resultant fluorescent ink compositions were printed on a white plainpaper substrate (a stock of standard copy paper lacking substantialbackground fluorescence), and allowed to dry. The print was invisible tothe unaided eye. The paper substrate was then exposed to UV light, andthe print appeared the appropriate color due to the fluorescence of thefluorescent ink composition used.

TABLE 4 FNC IC Binder <1% H₂O > 34%; PEG 40% glycerol 25% <1% H₂O > 58%;DMSO 40% glycerol 1% <1% H₂O > 58%; PEG 40% glycerol 1% <1% H₂O > 93%;DMSO 5% starch 1% <1% H₂O > 93%; DMSO 5% gelatin 1% <1% H₂O > 58%; DMSO40% gelatin 1%

EXAMPLE 3

For purposes of illustration, listed in Table 5 are various examples ofa fluorescent ink composition according to the present invention,wherein the fluorescent ink composition comprised functionalizedfluorescent nanocrystals, an ink carrier (“IC”), a binder, and asurfactant. For each composition illustrated in Table 5, functionalizedfluorescent nanocrystals were in a weight percent of less than 1%.Illustrative surfactants included triethylamine (“TEA”), and detergent(TRITON; “TTN”). The resultant fluorescent ink compositions were printedon a white plain paper substrate (a stock of standard copy paper lackingsubstantial background fluorescence), and allowed to dry. The print wasinvisible to the unaided eye. The paper substrate was then exposed to UVlight, and the print appeared the appropriate color due to thefluorescence of the fluorescent ink composition used.

TABLE 5 IC Binder Surfactant H₂O > 92%; DMSO 5% gelatin 1% 0.01% TEAH₂O > 92%; DMSO 5% gelatin 1% 0.1% TEA H₂O > 92%; DMSO 5% gelatin 1% 1%TEA H₂O > 88%; DMSO 5% gelatin 5% 0.01% TEA H₂O > 88%; DMSO 5% gelatin5% 0.1% TEA H₂O > 88%; DMSO 5% gelatin 5% 1% TEA H₂O > 92%; DMSO 5%gelatin 1% 0.01% TTN H₂O > 92%; DMSO 5% gelatin 1% 0.1% TTN H₂O > 92%;DMSO 5% gelatin 1% 1% TTN H₂O > 88%; DMSO 5% gelatin 5% 0.01% TTN H₂O >88%; DMSO 5% gelatin 5% 0.1% TTN H₂O > 92%; PEG 5% gelatin 1% 1% TEAH₂O > 92%; PEG 5% gelatin 1% 1% TTN

The foregoing description of the specific embodiments of the presentinvention have been described in detail for purposes of illustration. Inview of the descriptions and illustrations, others skilled in the artcan, by applying, current knowledge, readily modify and/or adapt thepresent invention for various applications without departing from thebasic concept, and therefore such modifications and/or adaptations areintended to be within the meaning and scope of the appended claims.

What is claimed is:
 1. A method of providing an image on a substrate forsubsequent identification comprising: applying onto the substrate in theform of an identifiable code pattern image, a fluorescent inkcomposition comprising one or more water soluble functionalizedfluorescent nanocrystals mixed in an ink carrier wherein each of the oneor more water soluble functionalized fluorescent nanocrystals in thecomposition is capable of fluorescing a specific color in providing theidentifiable code pattern image on said substrate which is multicolorupon subsequent identification.
 2. The method of claim 1 wherein theimage on the substrate is a security mark.
 3. A substrate having printedthereon an image according to claim
 1. 4. A substrate having printedthereon an image according to claim
 2. 5. A method for identifying anobject comprising the steps of: exposing the object comprising anidentifiable code pattern, formed from one or more water solublefunctionalized fluorescent nanocrystals mixed in an ink carrier, to anexcitation light source having a wavelength spectrum in a range of fromabout 300 nm to about 400 nm; and detecting a peak fluorescent emissionfrom the identifiable code pattern formed from the water solublefunctionalized fluorescent nanocrystals in a spectral range of fromabout 410 nm to 750 nm to identify the object in response to visualizingthe security mark.
 6. The method according to claim 5, furthercomprising: applying the identifiable code pattern to the object using aplurality of fluorescent nanocrystal compositions, wherein eachfluorescent nanocrystal composition comprises water solublefunctionalized fluorescent nanocrystals.